Defectivity and process control of electroless deposition in microelectronics applications

ABSTRACT

Methods and compositions for electrolessly depositing Co, Ni, or alloys thereof onto a substrate in manufacture of microelectronic devices. Grain refiners, levelers, oxygen scavengers, and stabilizers for electroless Co and Ni deposition solutions.

FIELD OF THE INVENTION

This invention relates to electroless plating of Co, Ni, and alloysthereof in microelectronic device applications. More specifically, thisinvention relates to additives including stabilizers, levelers, grainrefiners, and oxygen scavengers in Co and Ni electroless plating bathsfor microelectronic application.

BACKGROUND OF THE INVENTION

Electroless deposition of Co is performed in a variety of applicationsin the manufacture of microelectronic devices. For example, Co is usedin capping of damascene Cu metallization employed to form electricalinterconnects in integrated circuit substrates. Copper can diffuserapidly into a Si substrate and dielectric films such as, for example,SiO₂ or low k dielectrics. Copper can also diffuse into a device layerbuilt on top of a substrate in multilayer device applications. Suchdiffusion can be detrimental to the device because it can causeelectrical leakage in substrates, or form an unintended electricalconnection between two interconnects resulting in an electrical short.Moreover, Cu diffusion out of an interconnect feature can disruptelectrical flow. Copper also has a tendency to migrate from one locationto another when electrical current passes through interconnect featuresin service, creating voids and hillocks. This migration can damage anadjacent interconnect line and disrupt electrical flow in the featurewhere the metal migrates. Cobalt capping is employed to inhibit this Cudiffusion and migration.

Accordingly, among the challenges facing integrated circuit devicemanufacturers is to minimize diffusion and electromigration of metal inmetal-filled interconnect features. This challenge becomes more acute asthe devices further miniaturize, and as the features further miniaturizeand densify.

Another challenge in the context of metal interconnect features is toprotect them from corrosion. Certain interconnect metals, especially Cu,are more susceptible to corrosion. Copper is a fairly reactive metalwhich readily oxidizes under ambient conditions. This reactivity canundermine adhesion to dielectrics and thin films, resulting in voids anddelamination. Another challenge is therefore to combat oxidation andenhance adhesion between the cap and the Cu, and between structurelayers.

The industry has deposited Co-based caps over Cu and other metalinterconnect features, as discussed in, for example, U.S. Pat. Pub. No.2003/0207560 and U.S. patent application Ser. No. 10/867,346.

A particular Co-based metal capping layer employed to reduce Cumigration, provide corrosion protection, and enhance adhesion betweenthe dielectric and Cu is a ternary alloy including Co, W, and P. Anotherrefractory metal may replace or be used in addition to W, and B is oftensubstituted for or used in addition to P. Each component of the ternaryalloy imparts advantages to the protective layer.

A particular problem for the integration of this technology to currentULSI fabrication lines is high defectivity of the capping layer. Inrecent years, the defectivity has been an object in inventions relatingto plating baths and tools. See Katakabe et al. (U.S. Pat. Pub. No.2004/0245214), Kolics et al. (U.S. Pat. Pub. No. 2004/0134375), Dubin etal. (U.S. Pat. Pub. No. 2005/0008786), Cheng et al. (U.S. Pat. Pub. No.2004/0253814), Weidman et al. (U.S. Pat. Pub. No. 2005/0084615), Panchamet al. (U.S. Pat. Pub. No. 2005/0072525), and Saijo et al. (U.S. Pat.Pub. No. 2005/0009340). Defectivity reduction remains a challenge inULSI fabrication lines.

Typical defects in electroless plated cobalt alloys for use as caps oninterconnect features may be summarized as follows.

Nodulation: localized preferential growth or particle formation on theCu deposit, at Cu/dielectric and Cu/barrier interfaces, and ondielectric surfaces. This problem may be generally caused by a lack ofstability of the working bath, and formation of incubation centers inthe solution, such as Co³⁺ due to the oxidation of Co²⁺ by dissolvedoxygen.

“Grain decoration”: uneven morphology of electroless Co film along theCu line that replicates Cu erosion before plating and/or unevenly grownCo film due to initiation delay at Cu grain interfaces. Such growth cancontribute to overall deposit roughness.

Granularity: irregularly sized nanocrystallites and clusters ofamorphous electroless deposits of Co and its alloys with large grainsand well-defined grain interfaces. This type of morphology cancontribute to surface roughness.

Non-uniform growth: varying deposit thickness along the Cu substrate dueto different plating rate of electroless Co on different size features,features located in different areas, dense and isolated, and/or featureswith different surface areas.

Pitting: the formation of pits or pinholes due to localized incompleteCu surface coverage or extensive hydrogen bubble formation during thedeposition process of the electroless film.

Those defects decrease diffusion barrier effectiveness, lower thecapability of the capping layer to suppress electromigration, causeelectromigration failure, affect the signal propagation across thecircuitry, increase current leakage, and may even result in electricalshorts.

Therefore, a need continues to exist for substantially defect free,uniform, and smooth electrolessly deposited capping layers over Cuinterconnects.

SUMMARY OF THE INVENTION

Among the various aspects of the invention may be noted the provision ofmethods and compositions for Co and Ni electroless plating which yieldsubstantially defect free or very low defectivity alloys suitable forvarious applications in microelectronic devices. In one aspect, themethods and compositions are especially suitable for capping. The alloysare deposited from electroless deposition compositions comprising one ormore additives selected from among defect-reducing additives, includingstabilizers, levelers, grain refiners, and oxygen scavengers. Thisinvention does not require use of all these additives. And in differentaspects the invention is directed to these additives separately. Theseadditives could be defined as follows:

Stabilizers: additives for an electroless plating composition thatreduce spontaneous decomposition of the bath and uncontrollableprecipitation of the metal in the volume of the solution. For example,stabilizers suppress nodular, stray growth, and extensive hydrogenevolution, thereby preventing pitting.

Levelers: additives for an electroless plating composition that help toequalize the growth rate of the Co film on different Cu grains and at Cugrain interfaces, on different size features, features located indifferent areas, dense and isolated, and/or features with differentsurface areas. Thus, levelers promote formation of an electroless Cofilm with even morphology, low roughness, and uniform thickness of Co ondifferent Cu features.

Grain refiners: additives for an electroless plating composition thatpromote formation of the electroless film with uniformly sized grainsand smooth grain interfaces. Grain refiners, consequently, reducegranularity of the deposit and improve roughness.

Oxygen scavengers: additives for electroless plating composition thatchemically interact with dissolved oxygen to form inactive species,thereby consuming oxygen. Oxygen scavengers, in turn, inhibit oxidationof Co²⁺ to Co³⁺, inhibit incubation center formation in the solutionvolume, and affect nodulation on dielectric surface.

Briefly, therefore, the invention is directed to a method andcomposition for electrolessly depositing Co, Ni, or alloys thereof ontoa substrate in manufacture of microelectronic devices, the methodcomprising contacting the substrate with an electroless depositioncomposition comprising (a) a grain refiner comprising a grain refinercompound selected from the group consisting of a polyphosphoric acid anda polyphosphate salt, and (b) a source of deposition ions selected fromthe group consisting of Co ions and Ni ions.

The invention is also directed to a method and composition forelectrolessly depositing Co, Ni, or alloys thereof onto a substrate inmanufacture of microelectronic devices, the method comprising contactingthe substrate with an electroless deposition composition comprising (a)an oxygen scavenger selected from the group consisting of ascorbic acid,SO₃ ²⁻, HSO₃ ⁻, hydroquinone, catechol, resorcinol, hydrazine, andcombinations thereof and (b) a source of deposition ions selected fromthe group consisting of Co ions and Ni ions.

In another aspect the invention is directed to a method and compositionfor electrolessly depositing Co, Ni, or alloys thereof onto a substratein manufacture of microelectronic devices, the method comprisingcontacting the substrate with an electroless deposition compositioncomprising (a) a stabilizer selected from the group consisting of anaminobenzoic acid, an hydroxybenzoic acid, a molybdenum oxide, avanadium oxide, rhenium oxide, salts thereof, derivatives thereof, andcombinations thereof; and (b) a source of deposition ions selected fromthe group consisting of Co ions and Ni ions.

The invention is further directed to a method and composition forelectrolessly depositing Co, Ni, or alloys thereof onto a substrate inmanufacture of microelectronic devices, the method comprising contactingthe substrate with an electroless deposition composition comprising (a)a leveler selected from the group consisting of diphenyl oxidedisulfonic acids, triethanolamine salts of lauryl sulfate, ammoniumlaureth sulfates, alkylbenzene sulfonates, dodecylbenzene sulfonicacids, alkyldiphenyloxide disulfonate, low molecular weightpolypropylene glycol, and combinations thererof and (b) a source ofdeposition ions selected from the group consisting of Co ions and Niions.

Other objects and features of the invention will be in part apparent andin part pointed out hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B are AFM images showing Co alloy deposits preparedaccording to the method described in Example 8.

FIGS. 2A and 2B are AFM images showing Co alloy deposits preparedaccording to the method described in Example 9.

FIGS. 3A, 3B, 3C, and 3D are SEM photographs showing Co alloy depositsprepared according to the methods described in Example 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

In accordance with the invention, Co and Ni alloys are deposited usingmethods and compositions which yield a uniform deposit with excellentselectivity and enhanced smoothness and planarity. Additionally, thedeposit is substantially free of nodular growth and isolated alloyparticles on the dielectric. For example, a smooth electroless cap canbe electrolessly deposited over an interconnect feature in amicroelectronic device. The invention is described here in the contextof Co-based caps and Ni-based caps, but is also applicable to otherelectroless Co and Ni applications in the microelectronics industry.

The present invention stems from the discovery that depositing aCo-based or Ni-based cap from an electroless deposition compositionincorporating one or more of a variety of defect-reducing additivesincluding certain stabilizers, levelers, grain refiners, and oxygenscavengers improves the overall topography and morphology of thedeposit. Incorporating these various defect-reducing additives improvesthe bath stability, reduces the formation of particles on the dielectricsurface in between Cu features, improves surface roughness, controls andthereby reduces the formation of pits in the deposit, and refines thegrain size for better morphology, i.e., smaller grain size and anoverall amorphous deposit. An electroless plating composition of thepresent invention may comprise one, more than one, or all of the classesof defect-reducing additives, the classes being broadly defined asstabilizers, levelers, grain refiners, and oxygen scavengers.Classifying an additive into one of these four classes is not meant todelimit its function in the bath. It has been observed that many of theadditives have multiple functions in the bath, i.e., the additives mayfunction both as stabilizers and grain refiners, for example. Thefunction of each defect-reducing additive is directed broadly towardimproving the bath stability and reducing defects in the plated alloy.Additionally, the electroless deposition compositions of the presentinvention may include one or more of the additives from each class,i.e., the bath may include two or more stabilizers, for example.

Oxime and its derivatives, amino- and hydroxyl-benzoic acids andderivatives thereof, and refractory metal oxides when added to theelectroless deposition compositions of the present invention act asstabilizers.

Conventional stabilizers used in electroless Co and Ni plating includeheavy metals such as Pb and Cd or sulfur compounds such as thiourea andmercaptosuccinic acid. These stabilizers perform adequately forelectroless Co and Ni plating baths by reducing plating bathdecomposition. However, conventional stabilizers are disadvantageous foruse in electroless deposition compositions for depositing Co and Ni capsin microelectronic applications because the presence of thesestabilizers results in the co-deposition of heavy metals or sulfur intothe Co or Ni alloy cap. As such, in certain embodiments of theinvention, these conventional stabilizers are replaced with stabilizersselected from the group oxime and oxime derivatives, amino-substitutedand hydroxyl-substituted benzoic acids and derivatives thereof, andrefractory metal oxides.

Exemplary oxime-based compound stabilizers for use in these embodimentsof the present invention include ketoximes and aldoximes. Ketoximes arecommonly formed by a condensation reaction between ketones andhydroxylamine or hydroxylamine derivatives. Exemplary ketoximes includedimethylglyoxime, 1,2-cyclohexanedione dioxime, diphenylglyoxime, andpyridyl diglyoxime. Aldoximes are commonly formed by a condensationreaction between aldehydes and hydroxylamine or hydroxylaminederivatives. Exemplary aldoximes include salicylaldoxime andsyn-2-pyridinealdoxime. In the context of this description,“oxime-based” refers to compounds which comprise the functional group ofthe type formed by a condensation reaction between hydroxylamine or ahydroxylamine derivative and a carbonyl group, which carbonyl group maybe either a ketone or an aldehyde; including such compounds whetherformed by this condensation reaction or by some other mechanism, as itis the functional group, not the reaction mechanism, which is important.The structures of some oxime-based compound stabilizers are shown inTable I.

TABLE I Oxime-Based Compounds for Use as Stabilizers Name StructureDimethylglyoxime

Salicylaldoxime

1,2-Cyclohexanedione dioxime

syn-2-Pyridinealdoxime

Diphenylglyoxime

Pyridyl diglyoxime

Advantageously, when oxime-based compounds are added to Co-basedelectroless plating baths, the stabilizers reduce stray deposition of Coor Co alloys onto the dielectric and reduce the formation of Co-basednodules in the deposited cap. Without being bound to a particulartheory, it is preliminarily believed that the stabilizing capacity ofthese compounds may be related to their chelating strength, in thatoximes chelate metal ions in solution more strongly than the primarychelator, which may be, for example, citric acid. For example, dependingupon solution conditions, the log of the stability constant, k, of Cuwith dimethylglyoxime may be between about 9 and about 11. The log k ofNi with dimethylglyoxime may be between about 12 and about 17.Conversely, the log k of Cu with citric acid may be between about 4 andabout 6, and the log k of Ni with citric acid may be between about 4 andabout 6. Co, on the other hand, is still chelated by the primarychelator, citric acid. Dimethylglyoxime preferentially chelates metalimpurities such as Ni, Cu, and others and shifts their reductionpotentials, thus avoiding the tendency of localized nucleation andparticle formation. Excess amounts of dimethylglyoxime may furtherchelate with Co and affect the rate of Co deposition. However, becauseof the strong chelating effect, the plating bath is completelydeactivated when the concentration level reaches 200 ppm or higher.

In the baths of these embodiments of the invention, the concentration ofthe oxime-based compound stabilizer is between about 2 ppm to about 150ppm. Hereinafter, the term “ppm” shall refer to the concentration of anadditive in mass units of additive per mass units of plating solution.For example, 5 ppm shall mean 5 mg of the additive per kilogram ofplating solution. Because the density of the solution is approximately 1kg/L, a 5 ppm concentration is approximately 5 mg per Liter of platingsolution. Under such conditions, the oxime-based compound acts as a bathstabilizer and a leveler of the deposit.

Oxime-based compounds are added to the bath in a concentration range ofabout 2 ppm to about 150 ppm, preferably from about 5 ppm to about 50ppm, even more preferably about 5 ppm to about 20 ppm.

In another embodiment of the invention, the stabilizer may alternativelyor additionally include aminobenzoic acids, salts thereof, andderivatives thereof or hydroxybenzoic acids, salts thereof, andderivatives thereof. Exemplary aminobenzoic acids include 2-aminobenzoicacid (anthranilic acid, ortho-aminobenzoic acid), 3-aminobenzoic acid(meta-aminobenzoic acid), 4-aminobenzoic acid (para-aminobenzoic acid,PABA), and derivatives thereof, with 2-aminobenzoic acid currentlypreferred. Exemplary aminobenzoate salts include 2-aminobenzoate salts,3-aminobenzoate salts, 4-aminobenzoate salts, and derivatives thereof.The aminobenzoate salts may include counterions selected from amongsodium, potassium, and ammonium, preferably potassium and ammonium.Exemplary hydroxybenzoic acids include salicylic acid (2-hydroxybenzoicacid, ortho-hydroxybenzoic acid), 3-hydroxybenzoic acid(meta-hydroxybenzoic acid), 4-hydroxybenzoic acid (para-hydroxybenzoicacid), 2,3-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid,2,5-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid,3,4-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid,2,3,4-trihydroxybenzoic acid, 2,3,6-trihydroxybenzoic acid,2,4,5-trihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid,3,4,5-trihydroxybenzoic acid (Gallic acid), and derivatives thereof.Exemplary hydroxybenzoate salts include salicylate salts,3-hydroxybenzoate salts, 4-hydroxybenzoate salts, 2,3-dihydroxybenzoatesalts, 2,4-dihydroxybenzoate salts, 2,5-dihydroxybenzoate salts,2,6-dihydroxybenzoate salts, 3,4-dihydroxybenzoate salts,3,5-dihydroxybenzoate salts, 2,3,4-trihydroxybenzoate salts,2,3,6-trihydroxybenzoate salts, 2,4,5-trihydroxybenzoate salts,2,4,6-trihydroxybenzoate salts, 3,4,5-trihydroxybenzoate salts, andderivatives thereof. The hydroxybenzoate salts may include counterionsselected from among sodium, potassium, ammonium, preferably potassiumand ammonium. Exemplary aminohydroxybenzoic acid stabilizers are5-aminosalicylic acid, 2-amino-5-hydroxybenzoic acid(5-hydroxyanthranilic acid), and other hydroxyl-substituted anthranilicacids, which may be present as acids, salts thereof, or derivativesthereof. Derivatives of all of the above listed benzoic acid-basedstabilizers include those which have amino(—NH₂), nitro(—NO₂), andmethyl(—CH₃) or higher alkyl chain substituents on the aromatic ring.For example, an exemplary nitro derivative of salicylic acid may be3,5-dinitrosalicylic acid, which may be present as the acid or saltthereof. In the baths of the invention, the concentration of theseorganic acids may be between about 2 ppm and about 1500 ppm, such asbetween about 5 ppm and 1000 ppm. In one preferred embodiment it isbetween about 5 ppm and about 30 ppm, such as between about 20 and 30ppm. If the concentration is too low, the effect is not realized. If itis too high, initiation and growth are overly retarded. The structuresof some benzoic acid-based stabilizers are shown in Table II.

TABLE II Benzoic Acid-Based Stabilizers Name Structure 2-aminobenzoicacid(anthranilic acid, ortho-aminobenzoic acid)

3-aminobenzoic acid(meta-aminobenzoic acid)

4-aminobenzoic acid(para-aminobenzoic acid)

salicylic acid(2-hydroxybenzoic acid,ortho-hydroxybenzoic acid)

3-hydroxybenzoic acid(meta-hydroxybenzoic acid)

4-hydroxybenzoic acid(para-hydroxybenzoic acid)

2,3,6-trihydroxybenzoic acid

5-aminosalicylic acid

The stabilizing mechanism of the aminobenzoic and hydroxybenzoic acidsis believed to be related to the acid chelating strength. Without beingbound to a particular theory, it is believed that these compounds,similarly to the oximes, may preferentially chelate impurities such asCu and others introduced from raw materials or the substrate and therebysuppress their spontaneous deposition while leaving Co substantiallychelated by the weak primary chelator, which may be, for example, citricacid, and thus readily available for plating. For example, in bathswhere DMAB is used as a reducing agent, Cu ions introduced from Cuerosion during plating can cause particle formation or decomposition inthe bath. Particle formation may be avoided due to strong chelating ofthe organic acids for Cu ions. For example, the log of the stabilityconstant (log K or pK) of salicylic acid with Cu ions is about 11, whichis much higher than with Co ions (pK of about 6), while the pK for theprimary chelator, such as citric acid, with Co and Cu is around 4 to 6.Excess amounts of these stabilizers may further chelate with Co andaffect the initiation and growth rate of the alloy deposit. Because ofthe strong chelating effect, plating may be completely inhibited whenthe concentration exceeds 3000 ppm.

Advantageously, when the stabilizer is an oxime, an oxime derivative, anaminobenzoic acid, or a hydroxybenzoic acid, the stabilizer avoids bathdecomposition and co-deposition of undesired impurities in the Ni or Coalloy cap.

Exemplary refractory metal oxides for use as bath stabilizers in otherembodiments of the invention include molybdenum oxides, rhenium oxides,and vanadium oxides. Exemplary sources of molybdenum oxides includemolybdate salts such as MoO₃ predissolved with TMAH; (NH₄)₂MoO₄;(NH₄)₂Mo₂O₇; (NH₄)₆Mo₇O₂₄.4H₂O; (NH₄)₆Mo₈O₂₇.4H₂O; dimolybdates(Me₂Mo₂O₇.nH₂O); trimolybdates (Me₂Mo₃O₁₀.nH₂O); tetramolybdates(Me₂Mo₄O₁₃); metamolybdates (Me₂H_(10-m)[H₂(Mo₂O₇)₆].nH₂O; wherein m isless than 10); hexamolybdates (Me₂Mo₆O₁₉.nH₂O); octamolybdates(Me₂Mo₈O₂₅.nH₂O); paramolybdates (Me₂Mo₇O₂₂.nH₂O and Me₁₀Mo₁₂O₄₁.nH₂O);wherein in the above Me is a counterion selected from among ammonium,tetramethylammonium, and alkali metal cations and wherein n is aninteger having a value corresponding to a stable or metastable form ofthe hydrated oxide; molybdic acids; molybdic acid salts of ammonium,tetramethylammonium, and alkali metals; heteropoly acids of molybdenum;and other mixtures thereof. The concentration of molybdenum oxide isbetween about 10 ppm and about 800 ppm, preferably between about 10 ppmand about 30 ppm for low metal baths and between about 100 ppm and about500 ppm for high metal baths. At too low a concentration, the effect isnot realized. At too high a concentration, initiation and growth areadversely affected. Exemplary sources of vanadium oxides includevanadate salts such as triammonium salts, metavanadate salts such asammonium or sodium salts, pyrovanadates (V₂O₇ ⁴⁻), hexavanadates (HV₆O₁₇³⁻), V₂O₃, V₂O₄, and V₂O₅. Preferably, the vanadium oxides are selectedfrom among compounds which are easily dissolved in alkaline solution toform the needed oxide ions. The concentration of vanadium oxide isbetween about 1 ppm and about 100 ppm, preferably between about 2 ppmand about 5 ppm for low metal baths and between about 5 ppm and about 20ppm for high metal baths. Exemplary sources of rhenium oxides includerhenium trioxides, perrhenic acids, ammonium perrhenic acid salts,tetramethylammonium perrhenic acid salts, alkali metal perrhenic acidsalts, heteropolyacids of rhenium, and other mixtures thereof. Theconcentration of rhenium oxide is between about 2 ppm and about 300 ppm,preferably between about 5 ppm and about 10 ppm for low metal baths andbetween about 10 ppm and about 100 ppm for high metal baths.

Molybdenum oxide stabilizers appear to better enhance uniformity in thedeposit. At higher concentration levels, it is observed that theaddition of molybdenum oxides to the electroless bath reduces particleformation on the deposit and dielectric surfaces, thus improving theselectivity of the barrier.

As an additional advantage, when refractory metal oxides are used asstabilizers, reduction of the oxides may result in the co-deposition ofthe refractory metal into the Ni or Co alloy cap. The co-deposition ofMo from electroless baths comprising molybdenum oxides into the alloycap is especially high, ranging from about 0.5 atomic % to about 12atomic %. Advantageously, the thermal stability of the deposit isenhanced when Mo is co-deposited with W, a refractory metal commonlydeposited into Co-based alloys. Additionally, it is thought that Mo, Re,and V co-deposition into the alloy cap functions to increase corrosionresistance and diffusion resistance.

Levelers for use in the electroless deposition compositions of certainembodiments of the present invention include diphenyl oxide disulfonicacids such as Calfax 10LA-75, triethanolamine salts of lauryl sulfatesuch as Calfoam TLS-40, ammonium laureth sulfates such as Calfoam EA603, alkylbenzene sulfonates such as Calsoft L-40C and Calsoft AOS-40,dodecylbenzene sulfonic acids such as Calsoft LAS-99, alkyldiphenyloxidedisulfonate salts such as Dowfax 3b2, and soluble, low molecular weightpolypropylene glycol containing compounds such as PPG 425.

Conventional levelers used in electroless Co and Ni plating bathsinclude phosphate esters such as Polystep P-32 and Rhodafac RE-610 orEO/PO polymers such as Tetronic 304. These are used in most of theelectroless deposition compositions in the patent and scientificliterature. These levelers, which act as surfactants and surface tensionreducers, are thought to be necessary additives for Co and Nielectroless deposition compositions, especially for cappingapplications, to achieve satisfactory uniformity in the deposited alloycap. These additives improve the uniformity of the deposited alloy, butdisadvantageously have negative effects on selectivity, which in turnaffects the deposit morphology by increasing particle formation.

These conventional surfactants are substituted with the levelers listedabove in accordance with certain embodiments of the invention. Thelevelers of the invention, which act as surfactants and surface tensionreducers, are effective in reducing surface roughness and improvinguniformity in the deposited alloy, without the negative effect ofparticle or nodule formation. Without being bound to a particulartheory, the levelers of the present invention appear to better promotewetting of the metal interconnect surface, which thereby enhances thedeposition uniformity. The leveler may serve as a mild depositioninhibitor which can suppress three-dimensional growth to an extent,thereby improving morphology and topography of the film. The levelers ofthe present invention may also help refine the grain size, which yieldsa more uniform coating which has grain boundaries which are less porousto migration of Cu. In the baths of the invention, the concentration ofthe leveler may be between about 20 ppm and about 800 ppm, preferablybetween about 100 ppm and about 250 ppm. At too low a concentration, thedesired effect is not realized. At too high a concentration, there areincreased tendencies toward foaming, particle formation in the deposit,and lack of solubility.

In a different aspect, the invention is directed to grain refiners foruse in electroless deposition, including polyphosphoric acid orpolyphosphate salts. These grain refiners improve the deposituniformity, granularity of the deposit, and surface roughness. Sourcesof polyphosphoric acid and polyphosphate salts include ammoniumpolyphosphate and sodium polyphosphate, and others of the formulaM_((n+2))P_(n)O_((3n+1)), where M represents a counter ion includinghydrogen, and n is greater than or equal to three, for example, n=3 to600. These include tripolyphosphates, tetrapolyphosphates, etc., incontrast to the orthophosphates (P) and pyrophosphates (P-P). A suitablepolyphosphate structure is linear and unbranched as follows, where M isa H+ or a monovalent metal cation:

Other suitable polyphosphate structures are branched, such as in thefollowing fragment:

Some commercially available sources of polyphosphates and polyphosphoricacids include certain low quantities of ortho- and pyro- compounds.Accordingly, there are embodiments of the invention which usepolyphosphates or polyphosphoric acids as the grain refiner compounds,and ortho- and/or pyro- compounds are also present. And there areembodiments in which ortho- and pyro- phosphate and phosphoric acidcompounds are specifically excluded.

Some commercially available sources of polyphosphates and polyphosphoricacids include certain quantities of cyclophosphate (metaphosphate)compounds. These include compounds of the formula (MPO₃)_(n), where M isa H+ or a metal cation, and n=3 to 600. Examples includecyclotriphosphate and cyclotetratphosphate of the following structures(a) and (b), respectively:

In certain embodiments of the invention, the concentration of the grainrefiners may be between about 1 g/L and about 100 g/L, preferablybetween about 4 g/L and about 50 g/L. If the concentration is too low,the effect is not realized. If it is too high, there is increasedchelating of Co and therefore negative effect on plating rate. Withoutbeing bound to a particular theory, it is thought that polyphosphoricacid chelates cobalt ions present in the electroless bath. The additionof this second chelator to the bath changes the equilibrium of the bath,thus modifying the morphology of the deposit. That is, whenpolyphosphate is added to the bath, more phosphorous is co-deposited,resulting in smaller granularity and a more amorphous deposit, whichimproves the barrier property on Cu diffusion and thermal stabilityagainst Co or Ni recrystallization. Phosphorous contents in the depositare, for example, between 10 and 12 atomic % as compared to, forexample, 7 to 9% without the grain refiner. It is also observed thatmost of the formulations tested with the addition of polyphosphoric acidin the baths further prevent stray deposition on the dielectric. The rawmaterial sources can be from polyphosphoric acid or polyphosphate salts.

In a different aspect this invention is directed to oxygen scavengersfor use in certain embodiments of electroless compositions. Typicallyelectroless deposition compositions contain chelated Co²⁺ ions whensolution pH is higher than 7.5. In the presence of dissolved oxygen andchelator, Co²⁺ ions tend to oxidize to form Co³⁺ ions. Also, Cosolutions have poor shelf life due to the ease of oxidation of Co²⁺ toCo³⁺. Besides the storage conditions, the ease of oxidation of Co²⁺solution is formulation dependent. Degradation is faster if there is anammonium-containing component such as ammonia, DEA, TEA, or the like.Degradation is typically slower in non-ammonium systems. Co³⁺ decreasesbath activity, and, in extreme cases, may eventually prevent alloyplating. Additionally, Co³⁺ ions form low solubility salts withcomponents of the solution, causing the formation of particles in thebath. Conventionally, in order to extend the shelf life of the product,Co compositions were specially packaged, but the packaging methodsincreased the cost of the composition. A less expensive alternativebesides purging with inert gases, adding an oxygen scavenger to the Coelectroless deposition compositions of the present invention, has beenpresently discovered. A wide variety of oxygen scavengers may inhibit Cooxidation and improve the shelf life of the solutions, but the inventorshave discovered a select few which do not unduly interfere withdeposition initiation and/or selectivity. A preferred oxygen scavengerfor the electroless deposition compositions of the present invention isascorbic acid. Other preferred oxygen scavengers include sulfites suchas SO₃ ²⁻ and HSO₃ ⁻. Examples of SO₃ ²⁻ sulfites include sodiumsulfite, potassium sulfite, ammonia sulfite. Other organic chemicalssuch as hydroquinone, catechol, resorcinol, hydrazine are candidates forthis application. Besides shelf life improvement, the addition of theseadditives to the baths reduces the induction time of cobalt deposition,thus preventing Cu erosion from the plating solution during theinitiation period, which may cause grain decoration and pitting defects.In the baths of the invention, the concentration of ascorbic acid may bebetween about 0.1 g/L and about 5 g/L, preferably between about 0.5 g/Land about 2 g/L. The concentration of the sulfites may be between about0.01 g/L and about 1 g/L, preferably between about 0.01 g/L and about0.3 g/L. The concentration of the hydroquinone, catechol, resorcinol,hydrazine may be between about 0.01 g/L and about 0.5 g/L, preferablybetween about 0.01 g/L and about 0.1 g/L. The concentration is selectedto be at least enough to consume oxygen dissolved in the solution duringmanufacture and packaging, with some excess for bottle breathing. On theother hand, the concentration is selected not to be so high as to have anegative effect on plating. These additives can also be used in othernon-metal (Co or Ni) containing solutions as oxygen scavengers toimprove the plating bath performance.

The electroless deposition compositions of the present invention forelectroless plating of Co or Ni alloys such as in a metal capping layeronto a metal-filled interconnect may comprise one or more of theabove-identified classes of defect-reducing additives, includingstabilizers, grain refiners, levelers, and oxygen scavengers. Moreover,the compositions may comprise one or more defect-reducing additives fromeach class for additive effects. The compositions generally additionallycomprise a source of deposition ions, a reducing agent, and a complexingand/or chelating agent. The bath is buffered within a certain pH range.Optionally, the bath may also comprise a source of refractory ions.

For the deposition of a Co-based alloy, the bath comprises a source ofCo ions. In the context of capping of electrical interconnects, Co-basedalloys provide several advantages. They do not significantly alter theelectrical conductivity characteristics of Cu. Cobalt provides goodbarrier and electromigration protection for Cu. Cobalt, which isselected in significant part because it is immiscible with Cu, does nottend to alloy with Cu during assembly or over time during service. TheCo ions are introduced into the solution as an inorganic Co salt such asthe hydroxide, chloride, sulfate, or other suitable inorganic salt, or aCo complex with an organic carboxylic acid such as Co acetate, citrate,lactate, succinate, propionate, hydroxyacetate, or others. Co(OH)₂ maybe used where it is desirable to avoid overconcentrating the solutionwith Cl⁻ or other anions. In one embodiment, the Co salt or complex isadded to provide about 2 g/L to about 20 g/L of Co²⁺ to yield a Co-basedalloy of high Co metal content. In some applications, the Co content inthe electroless bath is very low, for example, as low as between about0.5 g/L and about 2.0 g/L of Co²⁺.

The bath may instead or additionally comprise a source of Ni ions, whichis introduced into the solution as an inorganic Ni salt such aschloride, sulfate, or other suitable inorganic salt, or a Ni complexwith an organic carboxylic acid such as Ni acetate, citrate, lactate,succinate, propionate, hydroxyacetate, or others. In one embodiment, theinorganic Ni salt is Ni(OH)₂. The Ni salt or complex is added to provideabout 1 g/L to about 20 g/L of Ni²⁺.

Depending upon the deposition mechanism and the desired alloy, thereducing agent is chosen from either a phosphorus-based reducing agentor a borane-based reducing agent. The deposition mechanism and thedesired alloy dictate the choice of the reducing agent. If an alloy isdesired which contains phosphorus, hypophosphite is chosen. If an alloyis desired which contains boron, a borane-based reducing agent ischosen, such as borohydride or a borane. Additionally, both phosphorousand borane-based reducing agents may be added to the plating bath.

Among the phosphorus-based reducing agents, hypophosphite is a preferredreducing agent in electroless plating films because of its low cost anddocile behavior as compared to other reducing agents. When hypophosphiteis chosen as the reducing agent, the finished alloy contains phosphorus.As is known, the plating solution requires an excess of H₂PO₂ ⁻ toreduce Co²⁺ into the Co alloy. As noted in Mallory and Hajdu, pp. 62-68,the molar ratio of Co ions to hypophosphite ions in the plating solutionis between 0.25 to 0.60, preferably between 0.30 and 0.45, for example.To ensure that a sufficient concentration of hypophosphite is present inthe plating bath for rapid initiation of plating and improved platingmorphology, the hypophosphite salt is added in an initial concentrationof about 2 g/L to about 30 g/L, for example about 21 g/L. Exemplaryhypophosphite salts include ammonium hypophosphite, sodiumhypophosphite, and potassium hypophosphite.

Hypophosphite reduces the metal ion spontaneously only upon a limitednumber of substrates, including Co, Ni, and Pd. Not included in thislist is Cu, which is a particular metal of interest for its use infilling interconnect features such as vias and trenches inmicroelectronic devices. For hypophosphite reduction over a Cusubstrate, the Cu surface must first be activated, for example, byseeding with the metal to be deposited (i.e., Co) by treating thesurface with a solution comprising a strong reducing agent such as DMABand ions of the metal to be plated (i.e., Co²⁺) or by seeding with acatalyst such as Pd.

Other preferred reducing agents include the borane-based reducingagents, such as borohydrides (sodium, potassium, cyano, trimethoxy, andtetramethylammonium, among others), monomethyl amine borane, isopropylamine borane, dimethyl amine borane (DMAB), diethyl amine borane (DEAB),trimethyl amine borane, triethyl amine borane, triisopropyl amineborane, pyridine borane, and morpholine borane. When a borane-basedreducing agent is chosen, boron becomes part of the plated alloy. As isknown, the plating solution requires approximately equal molar amountsof the borane-based reducing agent to reduce Co²⁺ into the Co alloy. Toensure that a sufficient concentration of reducing agent forself-initiated deposition is present in the plating bath, dimethyl amineborane, for example, is added in an initial concentration of about 0.5g/L to about 30 g/L, for example about 10 g/L.

Unlike hypophosphite, plating solutions with borane-based reducingagents do not need a copper surface activation step. Instead, thereducing agent catalyzes reduction of the metal ion onto the Cu surface.

Due to the oxidation of the reducing agent, P or B co-deposits with theCo or Ni. An effect of these elements in the deposit is to reduce grainsize, enhance amorphousness, which can render the microstructure moreimpervious to Cu diffusion and electromigration. For example, Co—W—Bwith high W content has an amorphous phase. Without being bound to aparticular theory, it is believed that the presence of refractory metaltogether with B and P improves the barrier properties by filling in thegrain boundaries of the crystalline structure of the deposit.

The bath further may contain agents for pH adjustment and bufferingagents. The bath pH is typically controlled by one or more pH adjustersand typically contains a pH buffer to stabilize the pH within thedesired pH range. In one embodiment, the desired pH range is betweenabout 7.5 and about 10.0. In one embodiment, it is between 8.0 up toaround 10. Exemplary agents for pH adjustment include potassiumhydroxide (KOH), tetramethylammonium hydroxide (TMAH),tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide(TPA), tetrabutylammonium hydroxide (TBAOH), MTEOH, ETEOH, BTEOH,ammonia, and other amines. Exemplary buffering agents include, forexample, borates, tetra- and pentaborates, phosphates, ammonia, andhydroxylamines such as monoethanolamine, diethanolamine,triethanolamine, and ethylenediamine, among others. The pH buffer levelis on the order of between about 2 g/L and about 50 g/L.

A complexing and/or chelating agent helps to keep Co ions in solution.Because the bath is typically operated at a mildly alkaline pH ofbetween about 7.5 and about 10.0, Co²⁺ ions have a tendency to formhydroxide salts and precipitate out of solution. The complexing agentsused in the bath are selected from among citric acid, malic acid,glycine, propionic, succinic, lactic acids, methanolamine (MEA),diethanolamine (DEA), triethanolamine (TEA), and ammonium salts such asammonium chloride, ammonium sulfate, ammonium hydroxide, pyrophosphate,polyphosphate, and mixtures thereof. Some complexing agents, such ascyanide, are avoided because they complex with Co ions too strongly andcan prevent deposition from occurring. The complexing agentconcentration is selected such that the molar ratio between thecomplexing agent and Co is between about 2:1 and about 10:1, generally.Depending on the complexing agent molecular weight, the level ofcomplexing agent may be on the order of between about 10 g/L and about200 g/L.

If desired, the plating bath may also include a refractory metal ion,such as W or Re, which functions to increase thermal stability,corrosion resistance, and diffusion resistance. Exemplary sources of Wions are tungsten trioxide, tungstic acids, ammonium tungstic acidsalts, tetramethylammonium tungstic acid salts, and alkali metaltungstic acid salts, phosphotungstic acid, silicotungstate, otherheteropolytungstic acids and other mixtures thereof. For example, onepreferred deposition bath contains between about 0.1 g/L and about 10g/L of tungstic acid. Other sources of refractory metal include rheniumtrioxides, perrhenic acids, ammonium perrhenic acid salts,tetramethylammonium perrhenic acid salts, alkali metal perrhenic acidsalts, heteropolyacids of rhenium, and other mixtures thereof.

Other additives, as are known in the art such as levelers, accelerators,and grain refiners may also be added. At low concentrations, hydrazinemay be added as a leveler, as disclosed in U.S. patent application Ser.No. 11/085,304. Levelers act with the stabilizer of the invention tofurther enhance deposition morphology and topography, and also tocontrol the deposition rate.

In some applications, the bath must be substantially sodium free, oralkali metal ion free.

Employing the foregoing baths, a variety of alloys can be deposited. Forexample, Co diffusion barrier layers include Co—W—P, Co—W—B, Co—W—B—P,Co—B—P, Co—B, Co—Mo—B, Co—W—Mo—B, Co—W—Mo—B—P, diffusion barrier layersinclude Ni—Co—P, Ni—Mo—P, Ni—Mo—B—P, Ni—Co—B, and Ni—Co—Mo—B—P, amongothers.

According to the practice of electroless deposition, a layer of Co or Coalloy may be deposited by exposure of the electroless platingcompositions to, for example, a patterned silicon substrate having viasand trenches, in which a metal layer, such as Cu, has already filledinto the vias or trenches. This exposure may comprise dip, floodimmersion, spray, or other manner of exposing the substrate to adeposition bath, with the provision that the manner of exposureadequately achieves the objectives of depositing a metal layer of thedesired thickness and integrity.

In applications where the invention is used for capping, surfacepreparation may be needed for removing organic residues left by CMP andfor dissolving Cu oxide from the Cu surface. Unless removed, the oxidecan interfere with adhesion of the cap and can detract from electricalconductivity.

Acidic pretreatment involves exposing the substrate to an acid selectedfrom among HCl, H₂SO₄, citric acid, methanesulfonic acid, and H₃PO₄ toremove CMP residues, Cu oxides, and Cu embedded in the dielectric byCMP. After the acidic pretreatment operation is completed, the substrateis rinsed by, e.g., DI water.

Alternatively or additionally, an alkaline pretreatment employs basiccleaner for removing oxide from the metal interconnect feature. Thiscleaner preferably removes all the oxide, for example copper oxide,without removing substantial amounts of the metallization in theinterconnects. Typical basic cleaners contain TMAH with addition ofhydroxylamine, MEA, TEA, EDA (ethylenediamine), or DTA(diethylenetriamine) at pH range of 9 to 12. A water rinse follows thealkaline pretreatment.

The electroless plating compositions according to the present inventionmay be used in conventional continuous mode deposition processes. In thecontinuous mode, the same bath volume is used to treat a large number ofsubstrates. In this mode, reactants must be periodically replenished,and reaction products accumulate, necessitating periodic removal of theplating bath. Preferably, in this mode, the bath contains an initiallyhigh concentration of metals ions for depositing onto the substrate.Alternatively, the electroless plating compositions according to thepresent invention are suited for so-called “use-and-dispose” depositionprocesses. In the use-and-dispose mode, the plating composition is usedto treat a substrate, and then the bath volume is directed to a wastestream. Although this latter method may be more expensive, the use anddispose mode requires no metrology, that is, measuring and adjusting thesolution composition to maintain bath stability is not required. It isadvantageous from a cost perspective to use lower concentrations ofmetal ions when working in “use-and-dispose” mode.

For auto-catalyzation of the electroless deposition, borane-basedreducing agents may be employed such as, for example borohydrides(sodium, potassium, cyano-, trimethoxy, and tetramethylammonium, amongothers), monomethyl amine borane, isopropyl amine borane, dimethyl amineborane (DMAB), diethyl amine borane (DEAB), trimethyl amine borane,triethyl amine borane, triisopropyl amine borane, pyridine borane, andmorpholine borane, mixtures thereof, or mixtures thereof withhypophosphite. Oxidation/reduction reactions involving the borane-basedreducing agents and Co or Ni alloy deposition ions are catalyzed by Cu.In particular, at certain plating conditions, e.g., pH & temperature,the reducing agents are oxidized in the presence of Cu, thereby reducingthe deposition ions to metal which deposits on the Cu. The process ispreferably substantially self-aligning in that the metal is depositedessentially only on the Cu interconnect. However, conventionalelectroless plating baths deposit a Co alloy that amplifies theroughness of the underlying Cu interconnect. In many instances, stray Cois deposited onto the dielectric. If the additives are added to theplating solution, as in the present invention, the electroless platingbath deposits a smooth and level Co or Ni alloy capping layer withoutstray deposition onto the dielectric.

As an alternative, certain embodiments of the invention employ anelectroless deposition process which does not employ a reducing agentwhich renders Cu catalytic to metal deposition. For such processes asurface activation operation is employed to facilitate subsequentelectroless deposition. A currently preferred surface activation processutilizes a Pd immersion reaction. Other known catalysts are suitable andinclude Rh, Ru, Pt, Ir, and Os. Alternatively, the surface may beprepared for electroless deposition by seeding as with, for example, Coseeding deposited by electroless deposition, electrolytic deposition,PVD, CVD, or other technique as is known in the art.

Plating typically occurs at a bath temperature of between about 50° C.to about 90° C. If the plating temperature is too low, the reductionrate is too low, and at a low enough temperature, Co reduction does notinitiate at all. At too high a temperature, the plating rate increases,and the bath can become too active. For example, Co reduction can becomeless selective, and Co plating may occur not just on the Cu interconnectfeatures of a wafer substrate, but also on the dielectric material.Further, at very high temperatures, Co reduction can occur spontaneouslywithin the bath plating solution and on the sidewalls of the platingtank. Plating rates achievable using the electroless depositioncompositions of the present invention may be between about 50 Å/minuteand about 300 Å/minute. Plating typically occurs for between about 1minute and about 3 minutes. As such, Co and Ni alloy capping layershaving thicknesses between 50 Å and about 300 Å are routinely achieved,which capping layers are substantially defect free, uniform, and smoothas electrolessly deposited.

Optionally, the capping layers can be subjected to a post depositioncleaning to improve the yield.

The following examples further illustrate the invention.

EXAMPLE 1 Electroless Deposition Composition Comprising Stabilizer forPlating Ternary Alloy Comprising Co—W—P

A first electroless plating bath was prepared comprising the followingcomponents:

-   -   20-40 g/L CoCl₂.6H₂O    -   40-80 g/L C₆H₈O₇ (citric acid)    -   10-30 g/L H₃BO₃ (boric acid)    -   4-12 g/L H₂WO₄ (tungstic acid)    -   10-20 g/L H₃PO₂ (hypophosphorous acid)    -   10-30 ppm CH₃C (═NOH)C (═NOH)CH₃ (dimethylglyoxime)

The electroless plating bath (1 L) was prepared at room temperature bypreparing two solutions separately, Solution A (500 mL) and Solution B(500 mL), and combining the solutions to yield the electroless platingbath.

To prepare Solution A (500 mL):

-   -   1. CoCl₂.6H₂O dissolved in water in an amount sufficient, upon        dilution to 500 mL, to yield a concentration which was twice the        final concentration.    -   2. Added citric acid and boric acid to the Co²⁺ solution.    -   3. pH adjusted to about 7.0 using TMAH.    -   4. Pre-dissolved tungstic acid in TMAH aqueous solution added to        the solution in an amount sufficient, upon dilution to 500 mL,        to yield a concentration which was twice the final        concentration.    -   5. Added surfactant if needed in an amount sufficient, upon        dilution to 500 mL, to yield a concentration which was twice the        final concentration.    -   6. pH adjusted to about 9.0 using TMAH.    -   7. Dimethylglyoxime added in an amount sufficient, upon dilution        to 500 mL, to yield a concentration which was twice the final        concentration.    -   8. pH readjusted to about 9.0 using TMAH    -   9. Diluted with water to 500 mL.    -   10. Filter to remove undissolved solids.

To Prepare Solution B (500 mL):

-   -   1. Hypophosphorous acid dissolved in water in an amount        sufficient, upon dilution to 500 mL, to yield a concentration        which was twice the final concentration.    -   2. Added citric acid and boric acid.    -   3. pH adjusted to about 9.0 using TMAH    -   4. Diluted with water to 500 mL.    -   5. Filtered to remove any solids.

To prepare the electroless plating bath

-   -   1. Combined 500 mL of Solutions A and B.    -   2. pH adjusted to between 8.0 and 9.5 with TMAH or citric acid,        if needed.

This bath was used to plate a smooth and uniform deposit of the Co—W—Pternary alloy at a plating bath temperature between 65° C. and 80° C.Plating was initiated by Pd seeding. Plating for about 1 minute yieldeda deposit having a thickness between about 100 Å and about 200 Å. Thus,the plating rate was between about 100 Å/min and about 200 Å/min. Thedeposited Co—W—P ternary alloy had an RMS between about 6 Å and about 10Å, which was only slightly higher than the substrate roughness.Importantly, the selectivity of the deposit was good, without particlesor stray deposition on the dielectric surface.

EXAMPLE 2 Electroless Deposition Composition Comprising Stabilizer forPlating Ternary Alloy Comprising Co—W—P

A second electroless plating bath was prepared comprising the followingcomponents:

-   -   20-40 g/L CoCl₂.6H₂O    -   40-80 g/L C₆H₈O₇ (citric acid)    -   10-30 g/L H₃BO₃ (boric acid)    -   4-12 g/L H₂WO₄ (tungstic acid)    -   10-20 g/L H₃PO₂ (hypophosphorous acid)    -   5-30 ppm C₇H₇NO₂ (2-aminobenzoic acid)    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of the Co—W—Pternary alloy at a plating bath temperature between 65° C. and 80° C.Plating was initiated by Pd seeding. Plating for about 1 minute yieldeda deposit having a thickness between about 100 Å and about 150 Å. Thus,the plating rate was between about 100 Å/min and about 150 Å/min. Thedeposited Co—W—P ternary alloy had an RMS between about 6 Å and about 10Å, which was only slightly higher than the substrate roughness.Importantly, the selectivity of the deposit was good, without particlesor stray deposition on the dielectric surface.

EXAMPLE 3 Electroless Deposition Composition Comprising Stabilizer forPlating Quaternary Alloy Comprising Co—W—Mo—P

A third electroless plating bath was prepared comprising the followingcomponents:

-   -   20-40 g/L CoCl₂.6H₂O    -   40-80 g/L C₆H₈O₇ (citric acid)    -   0-30 g/L H₃BO₃ (boric acid)    -   2-6 g/L H₂WO₄ (tungstic acid)    -   5-15 g/L H₃PO₂ (hypophosphorous acid)    -   200-300 ppm Calfax 10LA-75 (Pilot Chemical Co.)    -   50-250 ppm MoO₃ (Molybdenum oxide)    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of theCo—W—Mo—P quaternary alloy at a plating bath temperature between 65° C.and 80° C. Plating was initiated by Pd seeding. Plating for about 1minute yielded a deposit having a thickness between about 140 Å andabout 180 Å. Thus, the plating rate was between about 140 Å/min andabout 180 Å/min. The deposited Co—W—Mo—P quaternary alloy had an RMSbetween about 4 Å and about 8 Å, which was less than or equal to thesubstrate roughness. Importantly, the deposit uniformity across thedeposit was good, without skipped plating or thickness difference.

EXAMPLE 4 Electroless Deposition Composition Comprising Grain Refinerfor Plating Ternary Alloy Comprising Co—W—P

A fourth electroless plating bath was prepared comprising the followingcomponents:

-   -   3-10 g/L CoCl₂.6H₂O    -   10-30 g/L C₆H₈O₇ (citric acid)    -   0-10 g/L H₃BO₃ (boric acid)    -   0.2-2.0 g/L H₂WO₄ (tungstic acid)    -   0-10 g/L Polyphosphoric Acid 115    -   4-12 g/L H₃PO₂ (hypophosphorous acid)    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of the Co—W—Pternary alloy at a plating bath temperature between 65° C. and 80° C.Plating was initiated by Pd seeding. Plating for about 1 minute yieldeda deposit having a thickness between about 80 Å and about 120 Å. Thus,the plating rate was between about 80 Å/min and about 120 Å/min. Thedeposited Co—W—P ternary alloy had an RMS between about 4 Å and about 8Å, which was approximately equal to the substrate roughness.Importantly, the deposit had smaller grain sizes and better uniformity.

EXAMPLE 5 Electroless Deposition Composition Comprising Leveler forPlating Ternary Alloy Comprising Co—W—B

A fifth electroless plating bath was prepared comprising the followingcomponents:

-   -   20-30 g/L CoCl₂.6H₂O    -   30-60 g/L C₆H₈O₇ (citric acid)    -   10-20 g/L NH₄Cl (ammonium chloride)    -   2-8 g/L H₂WO₄ (tungstic acid)    -   5-10 g/L (CH₃)₂NHBH₃ (DMAB)    -   200-500 ppm PPG 425    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of the Co—W—Bternary alloy at a plating bath temperature between 55° C. and 95° C.Because DMAB was used as the reducing agent, no Pd seeding was necessaryto initiate deposition. Plating for about 1 minute yielded a deposithaving a thickness between about 150 Å and about 250 Å. Thus, theplating rate was between about 150 Å/min and about 250 Å/min. Thedeposited Co—W—B ternary alloy had an RMS between about 10 Å and about15 Å, which was slightly higher than the substrate. Importantly, theline roughness across different Cu grain faces decreased with theaddition of the leveler compared to with the deposit plated without theleveler in the bath.

EXAMPLE 6 Electroless Deposition Composition Comprising Oxygen Scavengerfor Plating Quaternary Alloy Comprising Co—W—B—P

A sixth electroless plating bath was prepared comprising the followingcomponents:

-   -   20-30 g/L CoCl₂.6H₂O    -   30-60 g/L C₆H₈O₇ (citric acid)    -   4-8 g/L H₂WO₄ (tungstic acid)    -   2-6 g/L (CH₃)₂NHBH₃ (DMAB)    -   5-15 g/L NH₄H₂PO₂ (Ammonium hypophosphite)    -   0.5-2 g/L Ascorbic acid    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of the Co—W—B—Pquaternary alloy at a plating bath temperature between 55° C. and 95° C.Because DMAB was used as the reducing agent, no Pd seeding was necessaryto initiate deposition. Plating for about 1 minute yielded a deposithaving a thickness between about 100 Å and about 150 Å. Thus, theplating rate was between about 100 Å/min and about 150 Å/min. Thedeposited Co—W—B—P quaternary alloy had an RMS between about 6 Å andabout 10 Å, which was only slightly higher than the substrate roughness.

EXAMPLE 7 Electroless Deposition Composition Comprising Stabilizer forPlating Quaternary Alloy Comprising Co—W—B—P

A seventh electroless plating bath was prepared comprising the followingcomponents:

-   -   3-10 g/L CoCl₂.6H₂O    -   10-30 g/L C₆H₈O₇ (citric acid)    -   0-10 g/L H₃BO₃ (boric acid)    -   0.2-2.0 g/L H₂WO₄ (tungstic acid)    -   4-8 g/L H₃PO₂ (hypophosphorous acid)    -   0.3-5 g/L (CH₃)₂NHBH₃ (DMAB)    -   0.2-5 g/L salicylic acid    -   pH adjusted to between 8.0 and 9.5.

One liter of this bath was prepared at room temperature according to aprotocol similar to that described in Example 1.

This bath was used to plate a smooth and uniform deposit of the Co—W—B—Pquaternary alloy at a plating bath temperature between 65° C. and 80° C.Because DMAB was used as the reducing agent, no Pd seeding was necessaryto initiate deposition. Plating for about 1 minute yielded a deposithaving a thickness between about 80 Å and about 120 Å. Thus, the platingrate was between about 80 Å/min and about 120 Å/min. The depositedCo—W—B—P quaternary alloy had an RMS between about 8 Å and about 12 Å,which was only slightly higher than the substrate roughness.Importantly, the bath stability was improved, resulting in less particleformation on the deposit.

EXAMPLE 8 Alloys Depositing from Electroless Deposition CompositionsWith and Without Stabilizer

To illustrate the advantages of the dimethylglyoxime stabilizer of thepresent invention, ternary alloys comprising Co, W, and P wereelectrolessly deposited from the electroless deposition composition ofExample 1 onto a starting silicon substrate. The substrate had exposedpatterned Cu wires embedded in Ta/TaN stack barrier surrounded withinterlevel dielectric (ILD) made of SiO₂-based material. The Cu wireshad a width on the order of 120 nm, and after CMP, the Cu surface waslower than the surrounding dielectric. The surface roughness was about 5Å.

The patterned Cu substrate was exposed to a preclean solution of 1%sulfuric acid to remove post-CMP inhibitor residues, copper (II) oxidelayer, and post-CMP slurry particles from ILD. It was then rinsed indeionized (DI) water, and subsequently activated with Pd.

To plate the alloy, the substrate was immersed in the Co—W—P electrolessdeposition composition of Example 1. The baths were kept at 75° C., at apH of about 9.0, and plating occurred for 1 minute.

Under experimental conditions, this bath plated a 180 Å thick Co—W—Palloy layer onto the copper substrate with a surface roughness of about8 Å. Thus, there was minimal increase in the surface roughness of theCo—W—P alloy layer plated with the bath of the present inventioncompared to the underlying copper substrate surface roughness.Additionally, the layer was substantially free of nodular growth at thelayer edges.

For comparison, a substrate was immersed in a comparative Co—W—Pelectroless deposition solution of Example 1 in all respects except withno dimethylglyoxime stabilizer. FIG. 1A is an image of the substrateplated without the dimethylglyoxime stabilizer, and FIG. 1B is an imageof the substrate plated with the dimethylglyoxime stabilizer. As can beseen in FIG. 1A, severe nodulation along the Cu/dielectric barrierinterface and particle formation on the dielectric surface are observedfor the cap plated from the dimethylglyoxime stabilizer-free bath. ThisCo—W—P capping layer has an increased risk of causing an electricalshort. The nodulation and particle formation is substantially reduced inthe Co—W—P capping layer deposited from the bath comprisingdimethylglyoxime. See FIG. 1B, which is an AFM image of a comb structureshowing very low density of nodules or particles on dielectric surfacesor along the Cu/dielectric barriers.

EXAMPLE 9 Alloys Depositing from Electroless Deposition CompositionsWith and Without Leveler

Improved surface uniformity and roughness may be achieved in a Co—W—Blayer deposited from an electroless plating bath comprising levelersacting as surfactants or surface tension reducers according to thepresent invention. FIGS. 2A and 2B are AFM images of a Co alloydeposited from electroless deposition compositions. FIG. 2A is an imageof a Co alloy deposited from a composition not comprising a leveler.FIG. 2B is an image of a Co alloy deposited from a compositioncomprising a leveler.

The more uniform and smoother surface of the Co—W—B layer shown in FIG.2B was deposited from a bath of Example 5 containing 500 ppm PPG 425.The mean surface roughness was approximately 8 Å, which is a minimalincrease in the surface roughness of the alloy layer plated with thebath of the present invention compared to the underlying coppersubstrate surface roughness (which was 6 Å). The Co—W—B layer shown inFIG. 2A was deposited from a plating bath not containing PPG 425. Themean surface roughness was approximately 13 Å, which is significantlyrougher than the underlying copper substrate surface roughness.

EXAMPLE 10 Alloys Depositing from Electroless Deposition CompositionsWith and Without Grain Refiner

Improved surface roughness and granularity may be achieved in a Co—W—Player deposited from an electroless plating bath comprising grainrefiners according to the present invention. FIGS. 3A, 3B, 3C, and 3Dare AFM images of a Co alloy deposited from electroless depositioncompositions. FIGS. 3A and 3B are images of a Co alloy deposited from acomposition not comprising a grain refiner. FIGS. 3C and 3D are imagesof a Co alloy deposited from a composition comprising a grain refiner.

The Co—W—P layer having improved surface roughness, significantlyreduced grain decoration, and improved granularity shown in FIGS. 3C and3D was deposited from the bath of Example 4 containing 10 g/Lpolyphosphoric acid. AFM data indicated the surface roughness may haveactually decreased compared to the underlying Cu substrate. The meansurface roughness was approximately 5 Å for 10 nm thick deposit comparedto the mean surface roughness of the Cu substrate of about 5 Å. Most ofthe baths tested having added polyphosphoric acid showed betterselectivity with clean dielectric surface, especially for low-k porousdielectric materials which tend to be problematic. The Co—W—P layershown in FIGS. 3A and 3B exhibits grain decoration and poor granularity.This alloy was plated from a bath which does not comprise the grainrefiner. The mean surface roughness for this layer was about 8 Å forabout 10 nm thick deposit, which is higher than the copper substrate(mean surface roughness of approximately 5 Å).

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. Forexample, that the foregoing description and following claims refer to“an” interconnect means that there are one or more such interconnects.The terms “comprising”, “including” and “having” are intended to beinclusive and mean that there may be additional elements other than thelisted elements.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

1. A method for electrolessly depositing Co, Ni, or alloys thereof ontoa substrate in manufacture of microelectronic devices, the methodcomprising: contacting the substrate with an electroless depositioncomposition comprising (a) a grain refiner comprising a grain refinercompound selected from the group consisting of a polyphosphoric acid, apolyphosphate salt, and a combination thereof wherein said grain refinercompound is a compound having a formula M_((n+2))P_(n)O_((3n+1)) or(MPO₃)_(n), where M represents a counter ion, and n=3 to 600; and (b) asource of deposition ions selected from the group consisting of Co ionsand Ni ions.
 2. The method of claim 1 wherein the grain refiner compoundis ammonium polyphosphate.
 3. The method of claim 1 wherein the grainrefiner compound is sodium polyphosphate.
 4. The method of claim 1wherein the grain refiner compound is polyphosphoric acid.
 5. The methodof claim 1 wherein the grain refiner compound is present in theelectroless deposition composition at a concentration between about 1g/L and about 100 g/L.
 6. The method of claim 1 wherein the grainrefiner compound is present in the electroless deposition composition ata concentration between about 4 g/L and about 50 g/L.
 7. The method ofclaim 1 wherein the electroless deposition composition further comprisesa reducing agent.
 8. The method of claim 1 wherein the electrolessdeposition composition further comprises an oxygen scavenger compoundselected from the group consisting of ascorbic acid, SO₃ ²⁻, HSO₃ ⁻,hydroquinone, catechol, resorcinol, hydrazine, and combinations thereof.9. The method of claim 1 wherein the electroless deposition compositionfurther comprises a stabilizer compound selected from the groupconsisting of an aminobenzoic acid, an hydroxybenzoic acid, a molybdenumoxide, a vanadium oxide, a rhenium oxide, salts thereof, derivativesthereof, and combinations thereof.
 10. The method of claim 1 wherein theelectroless deposition composition further comprises a leveler selectedfrom the group consisting of diphenyl oxide disulfonic acids,triethanolamine salts of lauryl sulfate, ammonium laureth sulfates,alkylbenzene sulfonates, dodecylbenzene sulfonic acids,alkyldiphenyloxide disulfonate, low molecular weight polypropyleneglycol, and combinations thereof.
 11. The method of claim 1 wherein thesubstrate is a copper metal layer located in a semiconductor integratedcircuit device.
 12. The method of claim 1 wherein the electrolessdeposition composition further comprises an oxime-based stabilizercompound selected from the group consisting of salicylaldoxime,syn-2-pyridinealdoxime, dimethylglyoxime, 1,2-cyclohexanedione dioxime,diphenyl glyoxime, pyridyl diglyoxime, and combinations thereof.
 13. Anelectroless plating solution for plating a metal capping layer onto ametal-filled interconnect in a microelectronic device, the solutioncomprising: a source of deposition ions selected from the groupconsisting of Co ions and Ni ions; a reducing agent; and a grain refinercompound selected from the group consisting of a polyphosphoric acid, apolyphosphate salt, and a combination thereof wherein said grain refinercompound is a compound having a formula M_((n+2))P_(n)O_((3n+1)) or(MPO₃)_(n), where M represents a counter ion, and n=2 to
 600. 14. Theplating solution of claim 13 wherein the grain refiner compound isammonium polyphosphate.
 15. The plating solution of claim 13 wherein thegrain refiner compound is sodium polyphosphate.
 16. The plating solutionof claim 13 wherein the grain refiner compound is polyphosphoric acid.17. The plating solution of claim 13 wherein the grain refiner compoundis present in the electroless deposition composition at a concentrationbetween about 1 g/L and about 100 g/L.
 18. The plating solution of claim13 wherein the grain refiner compound is present in the electrolessdeposition composition at a concentration between about 4 g/L and about50 g/L.
 19. The plating solution of claim 13 wherein the electrolessdeposition composition further comprises an oxygen scavenger compoundselected from the group consisting of ascorbic acid, SO₃ ²⁻, HSO₃ ⁻,hydroquinone, catechol, resorcinol, hydrazine, and combinations thereof.20. The plating solution of claim 13 wherein the electroless depositioncomposition further comprises a stabilizer compound selected from thegroup consisting of an aminobenzoic acid, an hydroxybenzoic acid, amolybdenum oxide, a vanadium oxide, rhenium oxide, salts thereof,derivatives thereof, and combinations thereof.
 21. The plating solutionof claim 13 wherein the electroless deposition composition furthercomprises a leveler selected from the group consisting of diphenyl oxidedisulfonic acids, triethanolamine salts of lauryl sulfate, ammoniumlaureth sulfates, alkylbenzene sulfonates, dodecylbenzene sulfonicacids, alkyldiphenyloxide disulfonate, low molecular weightpolypropylene glycol, and combinations thereof.
 22. The electrolessplating solution of claim 13 further comprising an oxime-basedstabilizer compound selected from the group consisting ofsalicylaldoxime, syn-2-pyridinealdoxime, dimethylglyoxime,1,2-cyclohexanedione dioxime, diphenyl glyoxime, pyridyl diglyoxime, andcombinations thereof.